This invention relates to the field of mechanical refrigeration which requires the periodic removal of frost from the evaporator heat transfer surfaces.
Methods which perform evaporator defrosting using refrigerant gas are well established by open-source technical publications. As stated by ASHRAE Handbook-Refrigeration-2010, Chapter 15: Retail Food Store and Equipment, compressor discharge gas or gas from the top of the warm receiver at saturated conditions flows to the evaporators requiring defrost. But during this process, the gas can condense to a liquid state and subsequently cause damage to the compressor. This persistent problem has been the attention of much patent activity but these efforts have lead to complex and ineffective solutions. Therefore the refrigeration industry still required a simple, reliable and cost-effective method of defrosting using refrigerant gas.
From a review of the technical literature and patent history, it appears two general concepts have been applied in an attempt to reduce compressor damage during gas defrost. One general concept has focused on methods for handling liquid refrigerant returning to the compressor, either by capture, diversion or re-evaporation. A clear example of this concept is presented by U.S. Pat. No. 4,318,277 to Cann et al which describes an accumulator for capturing liquid refrigerant returning to the compressor and then the utilization of hot gas from the compressor to vaporize the captured liquid refrigerant. And in similar fashion, U.S. Pat. No. 3,636,723 to Kramer explains the application of a heater for re-evaporating the captured liquid.
The second general concept has focused on methods for recirculating refrigerant vapor from the condenser to the evaporator while bypassing the expansion valve, thereby attempting to transfer heat from the ambient medium (typically the outside air) to the frost by using the compressor to recirculate refrigerant from the condenser to the evaporator. U.S. Pat. No. 2,069,201 to Allison describes the actuation of a bypass loop from the condenser to the evaporator but fails to assure that the loop contains only vapor prior to actuation. It is therefore believed that this method would result in substantial compressor damage. Likewise U.S. Pat. No. 5,065,584 to Byczynski explains an actuated recirculation loop from the condenser-to-evaporator-to-compressor but does not provide a means for sequestering liquid refrigerant that may reside within this loop prior to actuation. U.S. Pat. No. 2,688,850 to White and U.S. Pat. No. 3,098,363 to Shrader describe an alternate method of recirculating refrigerant vapor from the condenser to the evaporator which diverts a portion of the refrigerant flow from the condenser. This diversion, or condenser bypass, significantly reduces performance of the condenser and thus the condenser cannot achieve its full potential for heat transfer.
In summary, a review of technical literature and prior art shows that gas-defrost can be an effective means of removing frost from evaporators. Nevertheless, in its current form as shown by prior art, gas-defrost can still lead to compressor failure and there are opportunities for improving its effectiveness. Therefore, what is needed is a gas-defrost method which assures that liquid does not return to the compressor. What is further needed is a gas-defrost method which transfers the maximum amount of heat from the ambient medium to the frosted evaporator. In order to achieve commercial viability, what is yet further needed is an effective defrosting method which operates with minimal compressor power and can be easily and reliably implemented.